Hybrid electric vehicles can employ both an electric propulsion system and an internal combustion propulsion system to improve fuel economy and reduce carbon emissions. Electric propulsion can be performed by an electric drive system that can include a number of components, typically at least including a power conversion circuit and a motor. In this arrangement, the power conversion circuit can controllably transfer power from a power source to the motor to drive a load. A high-voltage battery can be used as a power source for the electric drive system. The power conversion circuit provides an interface between the high voltage battery and the electric motor, and can boost a direct current voltage provided by the battery to a higher voltage required to drive the motor for high-speed vehicle operations. When used to boost a voltage from an input side to an output side, the converter is referred to as a boost converter.
A power converter can also be used to step down or lower a voltage from an output side to an input side. For example, the higher voltage on a motor/generator side of a power converter can be stepped down to a lower voltage in order to charge a battery on an opposing side of the converter. In the field of hybrid vehicles, it is common practice to charge a battery through regenerative braking, in which the mechanical energy of the wheels is converted to electrical energy by a generator, or by a motor operating as a generator, and provided to the battery via the power converter. When used to step down or reduce a voltage, the converter is referred to as a buck converter.
While a single power converter can operate as both a boost converter and a buck converter, and therefore support bidirectional power flow, in hybrid vehicle applications, the power converter is typically unidirectional boost from battery side to motor side and unidirectional buck from motor side to battery side. A typical power conversion circuit can comprise a power source, such as a battery, a variable voltage converter (VVC), an inverter, and a machine, for example, a motor or generator. Generally the power conversion circuit for a hybrid vehicle is designed in such a manner that the voltage VI on the inverter or motor/generator side of the VVC must remain higher than the voltage VB on the battery side of the VVC. When the voltage VB becomes higher than VI, a loss of VVC control can result, allowing inrush currents to build up on a VVC inductor within milliseconds, and thereby trigger an undesired system shutdown by over-current protection mechanisms. Unfortunately, however, maintaining a VI that is greater than VB can distort current output under low speed driving conditions, which in turn can reduce vehicle control and degrade vehicle performance. The high VI condition can also increase switching power losses and limit inverter capabilities.
Because electric and hybrid electric vehicles use a battery to provide power for an electric motor, the battery must be recharged to remain effective as a power source. Typically, when the vehicle is operated at high speeds, a generator in the electric drive system provides energy to the battery. In addition, the battery can be recharged during regenerative braking when the vehicle's kinetic energy is converted to electrical energy and provided to the battery. However, because the VI of the VVC must be maintained at a higher state than the VB of the VVC, energy cannot be transferred to the battery anytime when the voltage VI is lower than VB.
Plug-In Hybrid Electric Vehicles (PHEVs) can be recharged using a home electrical outlet. A recharging unit can be connected to the battery and also plugged into a standard electrical outlet, allowing an operator to recharge the battery overnight or while the vehicle is parked. However, the PHEV recharging unit is bulky and relatively expensive. Furthermore, the recharging unit can only be employed while the vehicle is turned off and not in use.